This invention relates, in general, to semiconductor devices, and more particularly, to a heterojunction bipolar transistor.
Heterojunction bipolar transistors exhibit electrical characteristics which are advantageous over the electrical characteristics of homojunction bipolar transistors. Silicon-germanium heterojunction bipolar transistor processing is compatible with existing silicon processing. Thus, silicon-germanium heterojunction bipolar transistors are preferred over other heterojunction bipolar transistors. In particular, silicon-germanium heterojunction bipolar transistors exhibit high emitter injection efficiency, reduced charge storage in the emitter, reduced or eliminated hole injection into the emitter. These characteristics are obtained because of the bandgap differential between the silicon and silicon-germanium metallurgical junction.
A silicon-germanium heterojunction bipolar transistor of the prior art consists of an N-type silicon collector, a P-type silicon-germanium base, and an N-type polysilicon layer. The N-type dopant from the polysilicon is diffused into the base to form an emitter region. The problem with this structure is that boron diffuses into the polysilicon layer or arsenic diffuses into the silicon-germanium base during the formation of the emitter. This diffusion degrades the bandgap differential by moving the metallurgical junction from the silicon-germanium and polysilicon interface into either the polysilicon layer or into the silicon-germanium base. Thus, the advantageous electrical characteristics described above are not exhibited.
A way of maintaining the metallurgical junction at the silicon-germanium and polysilicon interface is to prevent the diffusion of boron or arsenic from the respective layers. A structure in which the polysilicon layer is used as the emitter would solve this problem because the diffusion of boron or arsenic can be prevented. However, in this structure, the interface between the polysilicon layer and the silicon-germanium layer is poor, which results in the transistor exhibiting poor electrical characteristics, such as high leakage.
A transistor which solves this interface problem has been disclosed by King et al, in an article entitled, "Si/Si.sub.1-x Ge.sub.x Heterojunction Bipolar Transistors Produced by Limited Reaction Processing," published in IEEE Electron Device Letters, Vol. 10, No. 2, on Feb. 1989. The use of a thick silicon emitter of approximately 4,000 angstroms in thickness, instead of the polysilicon emitter eliminates the interface problem, and maintains the bandgap differential between the silicon-germanium layer and the silicon emitter layer in this case. However, in this structure, the thick silicon emitter must be lightly doped to avoid breakdown voltage problems and to avoid high emitter-base capacitance. A lightly doped, thick emitter exhibits high emitter resistance. Thus, it would be desirable to fabricate a heterojunction bipolar transistor in which the bandgap differential is ensured and also where emitter resistance and capacitance is low.
Accordingly, it is an object of the present invention is to provide an improved heterojunction bipolar transistor.
Another object of the present invention is to provide a heterojunction bipolar transistor having low emitter-base capacitance and low emitter resistance.
A further object of the present invention to provide a heterojunction bipolar transistor in which the bandgap differential between silicon and silicon-germanium is maintained.